Block Storage

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Block storage provides raw, formatted storage volumes that attach to virtual machines. Unlike object storage (accessed via HTTP API), block storage appears as a device — /dev/xvda on Linux or E: on Windows — and is typically formatted with a filesystem.

Provider Comparison

Feature	AWS EBS	Azure Managed Disks	GCP Persistent Disks
Max volume size	64 TB	32 TB	64 TB
Max IOPS per volume	260,000 (io2 Block Express)	160,000 (Ultra Disk)	100,000 (pd-extreme)
Max throughput	7,500 MB/s	2,000 MB/s	1,200 MB/s
Snapshot capability	Incremental to S3	Incremental to Blob Storage	Incremental to Cloud Storage
Multi-attach	io1/io2 (read/write for cluster)	Shared disks (read/write)	Read-only

SSD vs. HDD

Block storage volumes come in two families:

SSD-backed

Type	Use Case	Max IOPS	Max Throughput
gp3 (AWS)	General purpose, boot volumes	16,000	1,000 MB/s
io2 (AWS)	Critical database workloads	260,000	7,500 MB/s
pd-ssd (GCP)	General purpose	30,000	400 MB/s
pd-extreme (GCP)	High-performance databases	100,000	1,200 MB/s

HDD-backed

Type	Use Case	Max IOPS	Max Throughput
st1 (AWS)	Streaming, big data, log processing	500	500 MB/s
sc1 (AWS)	Cold data, infrequent access	250	250 MB/s

Tip

For boot volumes, gp3 (AWS) or pd-ssd (GCP) provide excellent performance at low cost. Do not use HDD-backed volumes for operating system disks — boot performance will be poor.

EBS vs. Instance Store

AWS EC2 offers two types of block storage:

Attribute	EBS (Elastic Block Store)	Instance Store
Persistence	Survives instance stop/termination	Ephemeral — lost on stop/termination
Performance	Network-attached, variable	Physically attached to the host, maximum performance
Backup	Snapshots, replication	Must be managed externally
Use case	Persistent data, databases	Temporary data, caches, scratch space

Caution

Never store irreplaceable data on instance store volumes. If the instance stops, fails, or is terminated, all data on instance store volumes is permanently lost.

Encryption

Block storage encryption protects data at rest:

AWS EBS: Enable EBS encryption by default. Use KMS keys (customer-managed or AWS-managed).
Azure Managed Disks: SSE with platform-managed or customer-managed keys.
GCP Persistent Disks: Default encryption with CMEK or CSEK options.

Encryption is transparent to the instance — data is decrypted on read with no performance impact.

Snapshots and Backup

Snapshots capture the state of a block storage volume at a point in time:

Incremental: Only changed blocks are stored after the initial snapshot.
Cross-region: Copy snapshots to another region for disaster recovery.
Automated: Use AWS Backup, Azure Backup, or scheduled snapshots.

Restoring from a snapshot: Create a new volume from the snapshot. The volume is fully usable immediately, with background restoration of data blocks (lazy loading).

Choosing Between Object and Block Storage

Criteria	Object Storage (S3)	Block Storage (EBS)
Access pattern	HTTP API	Filesystem mount
Performance	~100-500 MB/s per partition	Up to 260,000 IOPS per volume
Max single file size	5 TB	Volume size limit
Concurrent access	Multiple readers/writers	Single instance (except multi-attach)
Cost	~$0.023/GB/month (Standard)	~$0.08/GB/month (gp3)
Use case	Static assets, backups, data lakes	Operating systems, databases, apps

Practical Example: Web Application Architecture

┌─────────────────────┐
│  Elastic Load Balancer  │
└─────────┬───────────┘
          │
┌─────────▼───────────┐
│  EC2 (web/app tier)  │
│  Boot volume: gp3    │
│  50 GB, 3000 IOPS    │
└─────────┬───────────┘
          │
┌─────────▼───────────┐
│  RDS (database tier) │
│  io2 volume: 500 GB   │
│  10,000 IOPS         │
└─────────────────────┘

The web/application servers use gp3 volumes — cost-effective with good performance for general workloads. The database uses io2 volumes with provisioned IOPS for consistent, predictable performance. Daily snapshots of the database volume enable point-in-time recovery.